1. Field of the Invention
The present invention relates to a system for measuring chromatic dispersion of an optical fiber and, more particularly, to a system for measuring a chromatic dispersion of an optical fiber based on a baseband phase comparison method.
2. Description of the Related Art
The chromatic dispersion characteristics of optical fibers are important for determining an information transmission speed of an optical fiber communication path. The following are the main methods used at present for measuring chromatic dispersion in single-mode optical fibers:
(1) A pulse delay time difference measurement method using a fiber Raman laser/spectroscope combination;
(2) A baseband phase comparison method using an LED (light-emitting diode)/spectroscope combination;
(3) A baseband phase comparison method using multiple LDs (laser diodes) of different wavelengths; and
(4) An interference method using the interference characteristics of light.
Of these methods, the baseband phase comparison method (3) will be described below. In general, optical signals having different wavelengths differ in group velocity due to material dispersion and waveguide dispersion, resulting in phase differences of the optical signals after their propagation through an optical fiber. The baseband phase comparison method utilizes this fact. Optical signal generators, e.g., LDs having different wavelengths are arranged in an optical signal transmitter, and two types of optical signals, i.e., a reference optical signal and a measurement optical signal, which are intensity-modulated by a sine wave modulation signal, are generated. The two types of optical signals are incident on a reference optical fiber and a measurement optical fiber, respectively. In an optical signal receiver, a group delay time difference is calculated from a phase difference between the wavelengths after propagation through the measurement optical fiber. The measurement result is approximated by appropriate function .tau.(.lambda.), and the function is analytically differentiated to obtain a target chromatic dispersion characteristics D(.lambda.)=d.tau.(.lambda.)/d.lambda.. The graph in FIG. 2 represents the relationship between the group delay time difference and the wavelengths.
In the baseband phase comparison method, when measurement is done for an installed optical fiber by the far-end method, it is required to conduct synchronization between the transmitter and the receiver. When the transmitter and the receiver are to be synchronized, a cable or radio communication means is provided between the transmitter and the receiver to allow communication therebetween. Thus, setting of the modulation signal and wavelengths, and setting of the local signal at the receiver are manually performed. However, if the numbers of frequencies and wavelengths of the modulation signal subjected to measurement are increased, the measurement operation is much complicated. Thus, measurement of the chromatic dispersion requires much time. Furthermore, since the parameters are manually set, they may be erroneously set. In addition, since such erroneous setting cannot be checked, the reliability of the measurement result cannot be determined.